When predators unexpectedly decrease instead of increase plant production

Area of Science:

• Ecology
• Ecosystem Services
• Predator-Prey Dynamics

Background:

• Predators provide ecosystem services, including regulating herbivores to enhance plant production.
• Grasshoppers are dominant herbivores in grasslands, with birds as their primary predators.
• The ecosystem service of predation is often assumed but requires specific conditions to manifest.

Purpose of the Study:

• To investigate the conditions under which bird predation on grasshoppers enhances grassland plant production.
• To determine the role of nutrient cycling, specifically nitrogen, in mediating this ecosystem service.

Main Methods:

• A multi-site, multi-year grassland field experiment was conducted.
• Birds were excluded from experimental plots to assess their impact on grasshopper populations and plant production.
• Nutrient availability and plant community composition were monitored.

Main Results:

• Bird exclusion led to increased grasshopper abundance in some sites.
• Plant production increased only when grasshoppers reduced nitrogen availability by selectively consuming fast-decomposing plant species.
• Grasshoppers more frequently increased nitrogen availability than decreased it, making the service inconsistent across sites.

Conclusions:

• The ecosystem service of bird predation enhancing plant production is contingent on grasshopper feeding behavior influencing nitrogen cycling.
• Grassland predator conservation benefits are not consistently realized due to variable interactions between herbivores, plants, and nutrient dynamics.
• Further research is needed to understand the factors driving grasshopper selectivity and its impact on nutrient availability.